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CRYSTAL OSCILLATOR WITH QUADRATURE OUTPUTS

IP.com Disclosure Number: IPCOM000005597D
Original Publication Date: 1986-Oct-01
Included in the Prior Art Database: 2001-Oct-18
Document File: 2 page(s) / 73K

Publishing Venue

Motorola

Related People

Will McKinzie: AUTHOR

Abstract

This article describes a novel circuit concept which may be employed to achieve dual outputs from acrystal oscillator, which are not only coherent, but have a 90 degree phase relationship.

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MOTOROLA Technical Developments Volume 6 October 1986

CRYSTAL OSCILLATOR WITH QUADRATURE OUTPUTS

by Will McKinzie

Abstract

This article describes a novel circuit concept which may be employed to achieve dual outputs from acrystal oscillator, which are not only coherent, but have a 90 degree phase relationship.

   Quadrature sinusoidal sources are necessary for modulation and demodulation in many communication systems. One popular way to generatetwo signals with a fixed 90 degree difference is to follow an oscillator with a hybrid 90 degree phase splitter as shown in figure 1.

However, reverse isolation between output ports b and c of the hybrid is only 3 db at its center frequency so buffer amplifiers 03 and Q4 are typically needed to allow outputs to appear independent of one another.

Circuit Description

By moving the 90 degree hybrid inside the loop of the oscillator as shown in figure 2 one can achieve a 90 degree phase split of signal and still satisfy the loop criteria of gain and phase necessary for oscillation.

   The hybrid is a four port device that provides a + 45 degree phase change from port a to port c. The pole produced by C, in parallel with a combination of R,, in parallel with R, provides 45 degrees of phase lag at the crystal's series resonant frequency. Qq and Qz are common base and common collector amplifiers, respec- tively, which contribute little phase shift. Hence, the net loop phase shift is zero degrees at resonance. Q, pro- vides most of...